1. Field of the Invention
This invention relates to a signal generating apparatus, and more particularly a QPSK (Quadrature Phase Shift Keying) signal generating apparatus.
2. Description of the Related Art
QPSK codes are used in a modulation system in which time-sequential data is divided into unit data strings and these unit data strings are converted into I-channel and Q-channel signals crossing each other at right angles, to express a state. With these code, it is possible to express four states according to combinations of (I, Q) in the I, Q plane: for example, (0, 0), (1, 0), (0, 1), and (1, 1) (refer to RCR-SRD-27, the Radio System Development Center Foundation).
To generate such a QPSK signal, for example, QPSK signal generators have been used. Time-sequentially supplied digital signals are converted by a mapping circuit into QPSK codes expressed by I and Q signals. The mapping circuit outputs I and Q signals in synchronization with a specific clock and these signals are supplied to a digital roll-off filter to prevent interference between QPSK codes. The output of the digital roll-off filter is converted by a digital-to-analog converter into an analog signal, which is supplied to a low-pass filter to attenuate undesired frequency components. The output of the low-pass filter is the QPSK signal.
The digital roll-off filter is generally realized by an FIR (finite impulse response) or IIR (infinite impulse response) digital filter. These digital filters are made up of digital adders, digital multipliers, or digital delay elements. The scale of these digital circuits is generally large, which is an obstacle to making the QPSK signal generator more compact.
To avoid the obstacle, a method of constructing a QPSK signal generator without using a large digital roll-off filter has been proposed. This method can produce only four kinds of QPSK signals determined by combinations of (I, Q). Thus, by preparing the impulse responses of a digital roll-off filter which correspond to the input signals beforehand, instead of supplying in real time the QPSK signals converted at the mapping circuit to the digital roll-off filter, and only when the input is arrived, supplying the impulse response corresponding to the input signal, the function of the digital roll-off filter can be achieved.
In this case, in order to make the impulse response corresponding to the input signal approximate thereto in real time, the result of adding impulse responses of digital roll-off filters to the extent that an effect on the output signal is sufficiently small, is actually outputted. Because the impulse responses of the digital roll-off filter are bits of digital data, this causes the problem that a large digital adder is required to add these bits.
As mentioned above, when a digital roll-off filter is used, it is difficult to make the entire size of the QPSK signal generator smaller because the digital roll-off filter is composed of a large digital circuit. Also, if a signal generator uses no digital roll-off filter, since it requires an adder for digital data, it is not suitable for making the QPSK signal generator more compact.
As described above, conventional QPSK signal generators need large digital filters. In the case where the function is modified without using digital filters, a large digital adder is required, too. To improve the accuracy of signal, it is necessary to use high-accuracy elements for D/A converter components.